1. Field of the Invention
The present invention relates to abrasive media and processes for manufacturing the abrasive media. The media are thin flexible abrasive sheeting used for lapping, polishing, it finishing or smoothing of workpiece surfaces. In particular, the present invention relates to such media used as removable or replaceable abrasive sheeting that are able to operate at high surface speeds, particularly media having an annular distribution of abrasive particles bonded in monolayers to the top surfaces of raised island shapes which are repeated in patterned arrays. Forming raised islands integrally attached to inexpensive backing sheets, precisely leveling the height of each island, resin coating the islands and applying abrasive particles to the resin economically creates an abrasive article which will grind a workpiece precisely flat and also generate a smooth workpiece surface. Coolant water freely passing through flow channels formed by the valley passageways between the raised islands flushes out grinding swarf and also minimizes hydroplaning of the workpiece.
2. Background of the Invention
High speed lapping and grinding using fixed abrasive on sheet disks for both rough grinding and smooth polishing is now a practical reality. Most performance issues relate to four primary concerns, 1) hydroplaning caused by water lubricant and 2) a free exit path for grinding debris swarf away from the contact area between the abrasive and the lapped article, 3) the utilization of all the abrasive particles attached to a sheet and 4) variations during abrading use created by thickness variations of abrasive disks along their tangential surfaces. Unique answers for all four problems of hydroplaning, debris path, use of particles and thickness variations have been defined and numerous solutions have been created.
This invention references commonly assigned U.S. Pat. Nos. 5,910,041; 5,967,882; 5,993,298; 6,048,254; 6,102,777; 6,120,352 and 6,149,506 and all contents of which are incorporated herein by reference.
The most serious problem remaining in the commercial use of high speed lapping and polishing processes is the availability of high quality abrasive article sheets that have certain important characteristics. The present invention describes sheets that can rapidly advance the use of high speed lapping by providing abrasive sheets that meet the needs of the technology. The sheets should be of a sufficient dimension (e.g., at least a 6 inch (15.3 cm) diameter, at least a 12 inch (30.5 cm) diameter, or at least an 18 inch (45.7 cm) or larger diameter, and have islands comprising abrasive particles (preferably secured to a substrate and preferably arranged in an annular band). The islands have an uppermost abrasive surface that is extremely flat and of uniform thickness. Conventional flat surface grinding or lapping platens are set up to use the full surface area of a circular shaped flat flexible sheet of abrasive. However, the abrasive contact surface speed of the rotating disk varies from a maximum speed at the outer radius to essentially mathematical zero at the innermost center at the disk (where the radius is zero). The grinding material removal rate is roughly proportional to the surface speed of the moving abrasive, so that most of the grinding or lapping action, and the most efficient grinding or lapping action occurs at the outer portion of a rotating disk. Not only is the inside portion of the abrasive disk not used to remove workpiece surface material, but also this portion of the abrasive is not worn down by the workpiece, resulting in a shallow, cone shape of the abrasive disk surface. This uneven wear continues with usage of the disk, with the cone angle progressively increasing to a sharper angle. This cone angle is translated to the surface of the workpiece that is intended for rigid axis lapping of a workpiece and prevents precision flatness grinding of the workpiece, transferring uneven surface contour to the workpiece surface. An effective answer to this uneven wear is to create an abrasive disk with a narrow annular band of abrasive material (at the outer edges of the annulus), allowing the abrasive to wear down more evenly across the full surface of the abrasive disk (which is essentially the annulus, not a continuous circular surface) as the disk is used. This type of media is not available commercially and probably would not be with present production methods. This is because the continuous method of manufacturing abrasive disks cannot technically or economically produce the necessary annular configuration.
Presently, an important method of manufacturing circular abrasive sheets is to coat a continuous web backing with diamond particles to form a coated sheet material and then to punch out round disks from the coated sheet material. Effectively, most of the expensive inner surface area of these disks is wasted. If a conventional coated disk is used with a platen having an outer raised annular ring, then all of the abrasive coated area located at a radius inside the ring is not used as it does not contact the workpiece surface.
Furthermore, it is not practical to punch out radial rings from a coated web sheet for a number of reasons. First, there is not necessarily a ready market for the smaller disk that remains left over from the center punch-out for the annular ring. Also, there is a large waste of coated web material left over between the circular disks that are cut out, even with proficient xe2x80x9cnestingxe2x80x9d of the circular rings. In addition, the extra flexible center-less annular abrasive ring not having backing on the inner radius when made of thin 0.005 inch (0.127 mm) thick polyester web has limited structural body strength for handling and mounting. The center-less ring cannot be practically used on a platen without creating many problems, including the problem that water and grinding swarf tend to collect under the inside radial edge of the loose annular ring sheet. Furthermore, round or bar raised-abrasive islands having a thin top coating of expensive diamond particles are needed to compensate for hydroplaning affects at high surface speed lapping. The only island type of abrasive media now available which can reduce hydroplaning is a diamond particle metal plated Flexible Diamond Products abrasive sheet supplied by the 3M Company (Minnesota Mining and Manufacturing Co.). However, due to the manufacturing process of this product, the product is commercially limited by at least two counts. First, each disk has large variations in flatness, or thickness, and, due to its unique construction, cannot be made flat enough to use effectively at high speeds where the unevenness is accentuated by the speed. Second, the Flexible Diamond Product abrasive sheet is constructed from plated diamonds, which have been unable to produce a smooth polished finish.
Another widely used product from 3M Company is the pyramid shaped Trizact abrasive, which helps with hydroplaning effects. However, it is only practical for this product to be created with inexpensive abrasive media such as aluminum oxide, which tends to wear fast and unevenly across its surface. Again, this is a continuous web type of product, which does to have the capability of having or maintaining precise abrasive thickness control.
Two common types of abrasive articles that have been utilized in polishing operations include bonded abrasives and coated abrasives. Bonded abrasives are formed by bonding abrasive particles together, typically by a molding process, to form a rigid abrasive article. Coated abrasives have a plurality of abrasive particles bonded to a backing by means of one or more binders Coated abrasives utilized in polishing processes are typically in the form of endless belts, tapes, or rolls which are provided in the form of a cassette. Examples of commercially available polishing products include xe2x80x9cIMPERIALxe2x80x9d Microfinishing Film (hereinafter IMFF) and xe2x80x9cIMPERIALxe2x80x9d Diamond Lapping Film (hereinafter IDLF), both of which are commercially available from Minnesota Mining and Manufacturing Company, St. Paul, Minn.
Structured abrasive articles have been developed for common abrasive applications. U.S. Pat. No. 5,152,917 (Pieper, et al.) discloses a structured abrasive article containing precisely shaped abrasive composites. These abrasive composites comprise a plurality of abrasive grains and a binder. U.S. Pat. No. 5,107,626 (Mucci) discloses a method of introducing a pattern into a surface of a workpiece using a structured abrasive article.
A new class of large diameter precise thickness disks that have an annular ring of raised islands coated with a thin coat of diamond abrasive particles is required for high speed lapping which requires a completely different manufacturing technique than has been employed in the past by the abrasives industry. The new batch type of processing required to produce these disks must be practical and cost effective. Eventually, this batch process of manufacturing a disk as a separate item should be converted partially or wholly into a continuous process when product sales volume demand warrants the investment in process equipment and converting technology.
The primary competitor for the sheet fixed abrasive polishing technology is slurry lapping, which is necessarily very slow, even though it has been progressively up-dated. Slurry lapping produces a flatter surface on a workpiece at the present time than can be accomplished by high speed lapping, which has limited the sale of the high-speed lapper machines. Other traditional grinding wheel machines can produce about the same flatness accuracy as the present configuration lapper but can not produce the associated smooth polish that typical workpiece parts require. Accurate flat and smooth surfaces are used on work piece component parts to prevent lubricating or other pressurized fluid leakage at the contact surface where these parts are mated stationary with other parts or where these parts are joined to dynamically rotate against each other.
High speed lapping uses expensive thin flexible abrasive coated disks that must be very precise in thickness and must also be attached to a platen that is very flat and stable. As the platen rotates very fast, this speed tends to xe2x80x9clevelxe2x80x9d the abrasive as it is presented to the workpiece surface. At high speeds only the high spots of the abrasive contact the workpiece, the remainder of the disk abrasive is not used until the high spots wear down. Thus, it is necessary for the total system to be precisely aligned and constructed of precision components to initialize the grinding. Furthermore, the wear of the abrasive must proceed uniformly across both the surface of the sheet and the surface of each island to maintain the required flatness of both the effective abrasive surface and correspondingly, the workpiece surface. These issues have all been addressed in the latest configuration of a lapper machine along with the process techniques employed in operating it. To generate even wear with rotating abrasive disks, an annular raised abrasive is used as taught in U.S. Pat. Nos. 5,910,041; 5,967,882; 5,993,298; 6,048,254; 6,102,777; 6,120,352 and 6,149,506. However, the desired large disks are not available, as the size of commercially available abrasive disks is presently limited to about 12 inches (30.48 cm) diameter. This severely limits the width of the annular ring without the resultant much slower surface grinding speed at the inside diameter of the ring. This slower speed also results in reduced material removal from the portion of the workpiece at this inside radial location. Furthermore, as the inside radial section of the abrasive disk wears slowly, the outside diameter portion progressively wears down faster which results in an uneven surface on the annular ring. Having larger nominal diameter abrasive disks with fairly narrow annular bands will inherently take care of most of these problems.
The typical workpieces that are lapped initially are not flat and have rough surfaces. Most potential customers seem to want both very flat (within 2 light bands) and smooth polished surfaces.
A preferred abrasive flat lapping process is now done in two separate steps. First, the parts are ground flat using a rigid spindle running at full 3,000 RPM speed, a very small contact force of 1 to 2 lbs. (0.454 to 0.908 kg) and typically, 3M""s 12 inch (305 mm) diameter metal plated diamond abrasive. Water flows between the round islands of abrasive, reducing hydroplaning. Hydroplaning typically produces a cone shaped ground surface. Second, parts are polished using a spherical action workpiece holder, with low to moderate contact forces of 2 to 15 lbs. (0.908 to 6.81 kg), and uses a smooth coated abrasive disk operating at lower speeds of about 1,000 RPM or less to prevent hydroplaning. At this time, no xe2x80x9cisland typexe2x80x9d of coated abrasive is available for polishing in combination with an effective polishing method.
Generally, use of the metal plated diamond island style abrasive disks to remove material is considered to be xe2x80x9cgrinding,xe2x80x9d as the surface finish is not smooth to the high standards of polishing. Use of the coated abrasives creates very smooth surfaces and is considered to be xe2x80x9clappingxe2x80x9d. The plated diamond disks tend to be very durable and may last a long time during use. The coated diamond and other abrasive particle disks are much more fragile and are consumed much more rapidly.
With respect to performance, with rigid flat grinding, 2 light bands of flatness are obtained which is not sufficiently flat for many applications. Polishing results in acceptable smoothness, but typically creates new problems with flatness because of hydroplaning. Flatness defects created in the polishing step include both cone shapes and saddle shapes.
It is important that super abrasives such as diamond (or other materials having minimal mohs hardness values within at least 20%, or at least 10% of the hardness of diamonds) be used at a minimum surface speed of 5,000 SFPM (surface feet per minute) or 1,524 meter per minute to achieve fast material removal.
The high surface speed of the plated abrasive island articles creates extraordinary high rates of material removal of very hard materials and this perhaps can be increased even further with higher speeds. This is the primary reason for the interest of the high speed grinding and lapping.
Hydroplaning of parts using fine small particle coated abrasive will always be a problem at very high speeds until an abrasive article disk is available which has xe2x80x9cislandsxe2x80x9d of abrasive which allows excess water to pass around the island edges. A recent new commercial form of abrasive disks that has the abrasive formed into small pyramids of abrasive is available and it initially works well from a hydroplaning standpoint when the pyramids are fresh and not too worn down. However, this Trizact(copyright) brand disk sold by 3M is created only with relatively soft aluminum oxide and tends to wear out fast. It is not logical that the manufacturer would use longer wearing diamond particles in these pyramid shapes, as each disk would consume so much diamond that the costs would be too high.
A number of inventions are beginning to be considered to address the desirability of using, abrasive islands to achieve acceptable lapper workpiece flatness but they have fundamental problems. In one example, island-like foundations, which are constructed of large diameter agglomerates comprising both abrasive particles and erodible filler materials, are used, but these large agglomerates tend to wear away at the abrasive article surface unevenly. In another example, abrasive articles with patterns of shallow sinusoidal shaped rounded island-like foundation ridge shapes, the ridges formed of filler materials, with abrasive particles coated conformally to both the peaks and valleys alike is described: the shallow ridge valleys are not necessarily oriented to provide radial direction water conduits on a circular disk for flushing grinding debris away from the work piece surface even prior to wear down of the ridges; and a substantial portion of the abrasive particles residing on the ridge valley floors remain unused as it is not practical to wear away the full height of the rounded erodible ridges to contact these lower elevation particles.
The preferred shape of a raised island abrasive article is rotational round disk with an outer annular ring of raised abrasive islands which can be manufactured in batches but the same raised island flexible backing material can be manufactured in continuous web form to create a linear article such as a rectangular sheet or a endless belt.
U.S. Pat. No. 5,611,825 (Engen) describes resin adhesive binder systems which can be used for bonding abrasive particles to web backing material, particularly urea-aldehyde binders. There is no reference made to forming or abrasive coating abrasive islands. He describes the use of make, size and super size coatings, different backing materials, the use of methyl ethyl keytone and other solvents. Loose abrasive particles are either adhered to uncured make coat binders which have been coated on a backing or abrasive particles are dispersed in a 70 percent solids resin binder and this abrasive composite is bonded to the backing. Backing materials include very flat and smooth polyester film for common use in fine grade abrasives which allow all the particles to be in one plane. Primer coatings are used on the smooth backing films to increase adhesion
U.S. Pat. No. 5,820,450 (Calhoun) and U.S. Pat. No. 5,437,754 (Calhoun) discloses the use of individual spaced truncated cones and rectangular agglomerate blocks attached to 50 micrometer (0.00196 inch) thick polyethylene terephthalate (PET) with an 18 micrometer (0.0007 inch) thick ethylene acrylic acid copolymer (EAA) surface primer coating using toluene to solvent viscosity-thin a abrasive slurry binder where the agglomerates are spaced with gaps on the backing by use of a embossed carrier web having spaced receptacles filled with the abrasive slurry mixture. U.S. Pat. No. 6,228,133 (Thurber, et al.) describes the application of silane coupling agent to abrasive particles which increases the adhesion of the particle to the binder and priming the backing surface for increased adhesion of the binder by corona discharge, ultraviolet light exposure, electron beam exposure, flame discharge and scuffing; abrasive particles are applied by electrostatic coating.
U.S. Pat. No. 4,311,489 (Kressner) discloses the use of irregular surface agglomerates of abrasive particles and a binder where the agglomerate binder is weaker than the agglomerate make coat binder to permit gradual wearing down of the agglomerate.
U.S. Pat. No. 5,219,462 (Bruxvoort, et al.) discloses the use of dot patterned recesses or through-holes in a backing sheet which are filled with a slurry of fine abrasive particles having an expanding agent which expands the slurry to rise above each recessed hole. The passageways between the raised abrasive composite dots can pass water and slurry until the dots arc worn down. A disadvantage with this type of abrasive article is that all of the abrasive particles contained in the recess hole at a location below the exposed surface of the backing sheet is lost for abrading use. The importance of the control of height of the top of the dot is recognized in the disclosure in that a flat mold surface can be pressed against the non-hardened abrasive dots but no description is presented concerning the importance and accuracy of controlling the dot heights.
U.S. Pat. No. 794,495 (Gorton) discloses dots of abrasive on round disks formed by depositing abrasive particles on adhesive binder wetted dot areas printed on the backing, primarily to aid the free passage of grinding debris away from the workpiece surface. These dot areas are not elevated as raised island shapes from the surface of the backing.
U.S. Pat. No. 1,657,784 (Bergstrom) discloses a variety of abrasive particle primitive shaped areas with space gaps between the abrasive areas to provide a passageway for grinding swarf.
U.S. Pat. No. 3,246,430 (Hurst), U.S. Pat. No. 2,838,890 (McIntyre) and U.S. Pat. No. 2,907,146 (Dyar) disclose the effect of an uneven abrasive surface on a workpiece article and various techniques to create separated areas of abrasives.
U.S. Pat. No. 5,549,961 (Haas, et al.) discloses abrasive particle composite agglomerates in the shape of pyramids, truncated pyramids, and beads which are mixed in a slurry having ultrasonic energy used to lower the slurry viscosity and vacuum to minimize air bubbles. Abrasive composites are forced with abrasive article surface densities of 700 to 7,500 mold cavities per square centimeter. A typical truncated pyramid has a height of 3.15 mils (80 micrometer), a base of 7.0 mils (178 micrometer) and a top of 2 mils (51 micrometer) and is continuously abutted with adjacent pyramids to form a flat continuous sheet of abrasive. When a xe2x80x9cdaisyxe2x80x9d form shape is cut out from a sheet, the daisy is flooded with water or water with additives including water soluble oils, emulsified oils, wetting agents which suggest low speed operation. Clay additives were used to improve the control of erodibility of the abrasive composite. Surface coatings including halide salts, metal oxides and silica were applied to the abrasive particles to increase adhesion.
U.S. Pat. No. 6,231,629 (Christianson, et al.) discloses a slurry of abrasive particles mixed in a binder to form truncated pyramids and rounded dome shapes on a backing. Fluids including water, an organic lubricant, a detergent, a coolant or combinations thereof results in a finer finish on glass. Fluid flow in valleys between the pyramid tops tends to produce a better cut rate and increased flatness during glass polishing. Abrasive diamond particles either have a blocky shape or a needle like shape and may contain a surface coating of nickel, aluminum, copper, silica or an organic coating.
U.S. Pat. Nos. 6,080,215 (Stubbs, et al.) and 6,277,160 (Stubbs, et al.) discloses side-by-side coatings of different size abrasive particles by use of abrasive coating slurries where the abrasive particles are surface coated with materials including coupling agents, halide salts, metal oxides including silica, refractory metal nitrides and carbides. Fillers including amorphous silica and silica clay are used in abrasive slurries which contain methyl ethyl keytone, MEK, and toluene, TOL in various mixture ratios. Drying patterns which can be seen visually and are referred to as Bernard cells alter the nature of the abrasive coating and their existence depends on airflow and heating conditions during thermal cure of the slurry binder. Polishing liquids used include lubricants, oils, emulsified organic compounds, cutting fluids and soaps.
U.S. Pat. No. 6,217,413 (Christianson) discloses use of phenolic or other resins where abrasive agglomerates are drop coated preferably into a monolayer of abrasive agglomerates and leveling and truing which levels or evens out the abrading surface is performed on the abrasive article resulting in tighter tolerance during abrading.
U.S. Pat. No. 5,910,471 (Christianson, et al.) discloses that the valleys between the raised adjacent abrasive composite truncated pyramids provide a means to allow fluid medium to flow freely between the abrasive composites contributes to better cut rates and increased flatness of the abraded workpiece surface.
U.S. Pat. No. 5,232,470 (Wiand) discloses raised molded protrusions of circular shapes composed of abrasive particles mixed in a thermoplastic binder attached to a circular sheet of backing.
U.S. Pat. No. 4,930,266 (Calhoun, et al.) discloses the application of spherical abrasive composite agglomerates made up of fine abrasive particles in a binder in controlled dot patterns where preferably one abrasive agglomerate is deposited per target dot by use of a commercially available printing plate. Small dots of silicone rubber are created by exposing light through a half-tone screen to a photosensitive silicone rubber material coated on an aluminum sheet and the unexposed rubber is brushed off leaving small islands of silicone rubber on the aluminum. The printing plate is moved through a mechanical vibrated fluidized bed of abrasive agglomerates which are attracted to and weakly bound to the silicone rubber islands only. The plate is brought into nip-roll pressure contact with a web backing which is uniformly coated by a binder resin which was softened into a tacky state by heat thereby transferring each abrasive agglomerate particle to the web backing. Additional heat is applied to melt the binder adhesive forming a meniscus around each particle, which increases the bond strength between the particle and the backing. The resulting abrasive has dots of abrasive particles on the backing but they are only raised away from the backing surface by the diameter of the abrasive agglomerates. Each abrasive agglomerate typically ranges in size from 25 to 100 micrometers and contains 4 micrometer abrasive particles.
U.S. Pat. No. 3,916,584 (Howard, et al.) and U.S. Pat. No. 4,112,631 (Howard) discloses the encapsulation of 15 micrometer and smaller diamond and other abrasive particles in spherical erodible composites as he discloses that large particles can be coated on abrasive articles or used in slurries without the need for encapsulation.
U.S. Pat. No. 6,186,866 (Gagliardi) discloses the use of protrusions having a variety of peak-and-valley shapes comprised of an erodible grinding aid where the protrusion shapes are surface coated with an adhesive resin and abrasive particles are drop coated or electrostatically coated onto the resin forming a layer of abrasive particles conformably coated over both the peaks and valleys. There are apparent disadvantages of this product. Only a very few abrasive particles reside on the upper most portions of the protrusion shaped peaks and this small fraction of the total number of particles coated on the surface will quickly be worn down or knocked off the peaks by abrading action due to their inherently weak resin support at the curved peak apex. As the abrading action continues with the wearing down of the erodible protrusions, more abrasive particles are available for abrading contact with a workpiece article but the advantage of the valleys used to channel coolant fluids and swarf has now diminished. The abrasive particles are very weakly attached to the sloping sidewalls of the protrusions due to the simple geometric vulnerability of bonding a separate particle to a protrusion wall side. Adhesive binder that does not naturally flow and surround the particle to generate substantial strength to resist abrading contact forces which will tend to leverage the particle and break it away from the wall. Much of the valuable superabrasive particles located in the valley areas are not utilized with this technique of particle surface conformal coating of peaks and valleys.
U.S. Pat. No. 5,190,568 (Tselesin) discloses a variety of sinusoidal and other shaped peak and valley shaped carriers that are surface coated with diamond particles to provide a passageway for the removal of grinding debris. The problems inherent with this technique include the change in localized grinding pressure, in newtons per square centimeter, when a work piece first contacts only a few abrasive particles located at the top of the peaks as compared to a greatly reduced pressure when the peaks are worn down and substantially more abrasive particle surface area is in contact with the workpiece. The inherent bonding weakness of abrasive particles attached to the sloping sidewalls is discussed and the intention for some of the lower abrasive particles located away from the peaks being used to structurally support the naturally weakly bonded upper particles. The material used to form the peaks is weaker or more erodible than the abrasive particles, which allows the erodible peaks to wear down, expose, and bring the work piece into contact with new abrasive particles. Uneven wear-down of the abrasive article will reduce its capability to produce precise flat surfaces on the work piece. Abrasive articles with these patters of shallow sinusoidal shaped rounded island-like foundation ridge shapes where the ridges are formed of filler materials, with abrasive particles coated conformably to both the ridge peaks and valleys alike is described. However, the shallow ridge valleys are not necessarily oriented to provide radial direction water conduits for flushing grinding debris away from the work piece surface on a circular disk article even prior to wear down of the ridges. Also, a substantial portion of the abrasive particles residing on the ridge valley floors remain unused as it is not practical to wear away the full height of the rounded ridges to contact these lower elevation particles.
U.S. Pat. No. 5,496,386 (Broberg, et al.) discloses the coating of a mixture of diluent particles and shaped abrasive particles on a make coat of resin where the function of the diluent particles is to provide structural support for the shaped abrasive particles.
U.S. Pat. Nos. 4,256,467 (Gorsuch) and 5,318,604 (Gorsuch. et al.) discloses abrasive articles where the coating of fibrous cloth at island areas built up in raised height by electroplating areas of the cloth positioned in contact with electrically insulated metal having arrays of exposed circular electrically conducting island forming areas. Abrasive particles contained in the electroplating bath are introduced to fall on the upper portion of the plated metal islands during the process of attaching them to the fiber islands. However, the particles do not lie in a common plane at a flat surface of the raised islands. Instead, the particles are attached at many different elevations within the island areas. This out of flatness occurs because the individual fibers of the cloth which support the build-up of plated metal to create raised island structures is not flat at the upper surface of the progressively built-up plated island due to the fibers being woven together to form the cloth material. The different height locations of the particles prevent the generation of precision smooth surfaces during the abrading action but the abrasive island articles are effective in producing flat work pieces. Another disadvantage of this product is that the plated cloth material must be stripped from the electrically conductive metal base and attached as a laminate with adhesive to a backing substrate to form an abrasive article. This laminated abrasive article structure does not have the precise thickness control due to thickness variations in both the island plated cloth material and the laminating adhesive film for effective utilization of the diamond abrasive particles for high speed lapping.
Lapper Process and Apparatus
Lapping or grinding with abrasives fixed to a flexible sheet is performed at high surface speeds of 3,000, 5,000 or 10,000 or more surface feet per minute (913, 1524, or 3,048 meters per minute), requiring the use of water-like lubricants to cool the workpiece and to carry away grinding swarf. A workpiece can be held rigidly or flexibly by a rotating spindle platen to effect grinding contact with a rotating abrasive platen, but the platen must be maintained precisely perpendicular to the abrasive surface to obtain a workpiece surface flat within about 2 lightbands. The aggressive cutting action of plated diamond island style flexible sheets requires the grinding contact perpendicular force to be near zero pounds at the start and end of the grinding procedure and to be controlled within plus or minus 0.5 pounds (227 grams) with a typical nominal force of 2.0 lbs. (0.908 kg) for an annular ring shaped workpiece having approximately 3.0 square inches (58.1 square cm) of surface area. Hydroplaning of the workpiece on the water lubricated abrasive is minimized when using abrasive covered raised island sheets, but is severe for uniformly coated abrasive disks generally used for smooth polishing or lapping. Hydroplaning causes cone shaped ground workpiece surfaces, even with raised platen annular rings. The abrasive platen must be ground very flat and the abrasive disk sheet must be precise in thickness to be used effectively at high speeds.
Abrasive disks of large 18 inch (0.457 m), 24 inch (0.609 m), 36 inch (0.914), 48 inch (1.22 m) or even 60 inch (2.3 m) diameter having an outer annular band of raised islands which have a thin precise coating of diamond particles can be produced effectively with very precise thickness control. Raised island foundation bases can be deposited on a backing by a variety of means on a variety of commonly available thin flexible plastic or metal backing materials. These island foundation base plateau surfaces are machined or ground after attachment to the backing to establish a precisely controlled thickness relative to the bottom surface of the disk backing material. It is not critical that the thickness of the backing be accurately controlled as with traditional precision backing for lapping abrasive articles as the islands are machined to a uniform height after they are deposited on the backing. Loose diamonds or other abrasive particles, including composite structured agglomerates, can be metal plated or organic resin binder coated as a single mono layer on top of the top flat surface of the islands. Abrasive particles can be attached to or drop coated or electrostatic coated onto a wet organic resin island surface coating. Abrasive particle slurry binders can be coated onto the island surfaces. Resin coatings are based on organic resins including phenolics and epoxies which have been used traditionally in the abrasive industry for many years. A make binder resin coating (is a batch coating for applying resin) can be applied to an island foundation top surface, abrasive particle powder applied, a partial or full resin cure effected, a resin size coat applied and then a full resin cure effected by heat or other energy sources. These disks principally would be produced by a batch process, but a more traditional continuous web process can also employ the same basic process technology of creating abrasive particle coated raised islands in array patterns where this abrasive web material can be converted to form annular disks or rectangular sheets or continuous belts or other abrasive articles such as daisy wheels. A wide range of abrasive articles can be produced with fine abrasive particle disk sheets or belts can be used for lapping and coarse particle disks can be used for grinding. All the abrasive articles can be used at high surface speeds, which fully utilize the increased abrading material removal rates which occur at high speeds, particularly with diamond particles.
A number of techniques are described to establish a uniform thickness of a make coat of binder to the top surface of island foundations which have been previously ground to a very precise height as measured from the bottom side of a backing material. One method to produce this make coat is to first spin coat a layer of binder resin onto a flexible sheet of backing and then to press this binder wetted coating onto the top surface of an annular array of raised islands attached to a round backing. Approximately one half (e.g., between 20% and 75%, or more) of the spin-coated binder is transfer coated to the island top surfaces when the spin-coated transfer sheet is separated from the island sheet. Abrasive particles can be drop coated onto the binder-wetted surface of the islands and then the binder can be partially or fully cured. Make coats of resin may be wet through the full thickness of the resin coat or only the surface of a partially cured resin may be given a wet surface condition by the application of heat or by other means prior to the application of abrasive particles. Subsequently, other size coats of binders can be applied to the island sheet, optionally coating either the island tops only, or covering both the island tops and the island valleys. Make coatings can be applied optionally by various printing techniques directly on the surface of the island both for the make coat, the size coat and other coatings. A variety of techniques are described which control the application of the abrasive particles to achieve a uniform density of particles on the surface of the islands where there is no more than 65 percent of a given island area surface covered by abrasive particles. Further, the resultant layer of particles is controlled to minimize the occurrence of more than a single (mono) layer of particles on an island surface. The resultant sheet or disk form of abrasive article has a single layer of abrasive particles bonded to island surfaces where the variation of height (measured from the backside of the abrasive particle backing) of adjacent particles on islands is typically less than one half the average diameter of the particle. One objective in the use of a single layer of abrasive particles is to utilize a high fraction of the expensive particles particularly the two superabrasives diamond and cubic boron nitride (CBN). Another objective is to minimize the dimensional change in the flatness of the abrasive article due to wear-down. A preferred abrasive particle size is 30 microns (micrometers) which is slightly more than 0.001 inch (25.4 microns). When the abrasive is fully worn away, the abrasive surface of the islands has therefore only changed by approximately 0.001 inch or 25.4 microns which is a very small change in height or flatness compared to other lapping abrasive articles in common commercial use at the present. A number of the present commercial articles are coated with fused spheres, pyramids and other agglomerate shapes which have nominal effective diameters of two to ten times or more, of the basic abrasive particles contained in the erodible agglomerate carrier shapes. These large agglomerates tend to wear unevenly from the contact with workpiece articles due both to the contact size of the workpiece typically being smaller than the abrasive article surface, and also, due to the increased wear-down at the outer diameter of an circular abrasive disk article and decreased wear-down at the slower surface speed movement at the inside diameter. When the agglomerate wears down unevenly on a portion of its surface, this uneven abrasive surface is now an presented to a new (sequential operation) work piece article which reduces the capability of the lapping process to quickly and economically effect the creation of a flat surface on the workpiece. The workpiece may be smoothly polished due to the characteristics of the fine abrasive particles imbedded in the erodible agglomerates, but the workpiece surface will tend not to be flat.
It is preferred that a single or monolayer of individual abrasive particles, such as natural or man-made diamond particles, be coated on abrasive island tops but a single or mono layer of erodible agglomerates made up of smaller abrasive particles can be used on top of the abrasive islands. In this work, each of the island foundations are high enough from the surface of the abrasive article backing that cooling water and generated grinding swarf can freely travel down the valleys between the island tops. The radial flow of water and debris swarf is created by the centrifugal forces generated by rotation of the abrasive sheet so the spent water exits the active grinding surface area of the disk while fresh clean water is supplied continuously over the whole time of the grinding event.